P0400: Exhaust gas recirculation (EGR) flow malfunction
The ECU sees that EGR valve flow is not what it expects. It is usually carbon build up in the valve or passages, or a control problem.
| Code | P0400 |
|---|---|
| System | Engine and transmission |
| Group | P04xx, Emissions control |
| Type | Generic: it means the same on every make |
| Can you keep driving? | With care |
What P0400 means
The EGR valve feeds some exhaust gas back into the intake to lower combustion temperature and emissions. The ECU compares what it asks for with what it measures, and the figures do not match. The code does not say whether flow is too low or too high, only that something is wrong.
Most common causes
- EGR valve dirty or stuck with carbon
- Blocked EGR passages or cooler
- Cracked or loose vacuum hose to the EGR (vacuum operated valves)
- Damaged connector or wiring on the valve
- Faulty EGR valve or its position sensor
- Faulty vacuum solenoid controlling the EGR
Symptoms
- Engine warning light on
- Hesitation or rough idle
- Loss of power
- More smoke on diesels
Can you drive with P0400?
With care You can drive, but the engine may run worse and go into limp mode. Have it checked soon so it does not get worse.
Open the bonnetthe in-depth explanation
How to check it
With your reader, watch the commanded and actual EGR position live if available: if they differ, the valve is not responding. On engines with an airflow meter, compare air flow with the EGR open and closed.
How to read and clear P0400 with Final OBD
Plug an OBD2 scanner in under the steering wheel, switch the ignition on and open Final OBD. Under Faults you’ll see P0400 with its description and whether it’s confirmed, pending or permanent.
If the engine stores it over standard OBD, with FREE you read this code with its explanation for nothing and clear it. With PRO you also read the faults of every control unit and clear them in all of them.
Clearing the code only turns the light off: if the cause is still there, it comes back. Clear it once it’s fixed, and not right before an inspection. It’s all in how to read and clear fault codes and in inspections and OBD.
Similar codes
- P0401 Exhaust gas recirculation (EGR) flow insufficient
- P0402 Exhaust gas recirculation (EGR) flow excessive
- P0403 EGR control circuit malfunction
- P0404 EGR circuit range or performance problem
- P0405 EGR sensor A circuit low
- P0406 EGR sensor A circuit high
- P0407 EGR sensor B circuit low
- P0408 EGR sensor B circuit high
To understand it properly
We explain how that system works, with drawings and no waffle, in our mechanics guide: EGR and particulate filter.
What does code P0400 mean?
The EGR valve feeds some exhaust gas back into the intake to lower combustion temperature and emissions. The ECU compares what it asks for with what it measures, and the figures do not match. The code does not say whether flow is too low or too high, only that something is wrong.
Can you drive with P0400?
You can drive, but the engine may run worse and go into limp mode. Have it checked soon so it does not get worse.
How do you clear P0400?
With an OBD2 scanner and an app like Final OBD, which clears it for free if it’s stored by the engine control unit. But clearing it only turns the light off: if the cause isn’t fixed, P0400 comes back.
Read it on your car
Final OBD reads and clears engine faults through standard OBD for free, each one explained in plain English. With PRO it also reads the faults of every control unit, each make’s own codes included, and clears them in all of them.
